The present invention relates to a polishing apparatus which can polish a wide-area subject surface of a thin-plate-like object such as a semiconductor wafer or a glass plate for an LCD with a high degree of uniformity in the subject surface that is as close to the uniformity of an ideal surface standard as possible.
First, a conventional polishing apparatus will be described with reference to FIG. 1. FIG. 1 is a sectional view with portions in elevation showing part of a conventional polishing apparatus in a polishing operation.
A polishing apparatus 1 having a configuration shown in FIG. 1 is typically used to polish a thin-plate-like object such as a semiconductor wafer (in the following description, a semiconductor wafer will be used consistently as an example of a thin-plate-like object to be polished). The polishing apparatus 1 is generally composed of a polishing surface table 2, a polishing head 5, and an abrasive supply hose 10.
For example, the polishing surface table 2 has a surface whose diameter is two times or more larger than that of a semiconductor wafer S. A polishing pad 3 made of, for example, polyester resin nonwoven fabric is fixed, that is, bonded, to the surface with an adhesive, for example, with a double-sided adhesive tape. The polishing surface table 2 is so configured as to rotate about a rotation axis 4 in a direction indicated by arrow Ra at 30 rpm, for example.
The polishing head 5 is fixed to a cylinder 7 via a rotary shaft 6 and disposed above the polishing surface table 2. The polishing head 5 is coaxial with the rotary shaft 6 and is rotated about an axis that is distant from the rotation axis 4 of the polishing surface table 2, for example, about an axis that is located at the midpoint of the radius of the polishing surface table 2, in the same rotation direction (indicated by arrow Rb) as the polishing surface table 2 at, for example, 30 rpm (i.e., at the same rotation speed as the polishing surface table 2).
An adhesion film 8 for holding the semiconductor wafer S, which also serves as a buffer member, is fixed to the bottom surface of the polishing head 5. To prevent the semiconductor wafer S from going out of place, a retainer ring 9 made of an epoxy resin or a polyacetal resin whose inner diameter is slightly larger than the outer diameter of the semiconductor wafer S is fixed to a peripheral portion of the surface of the adhesion film 8.
The abrasive supply hose 10, which is connected to an abrasive liquid supply apparatus, supplies an abrasive liquid L to the surface of the polishing pad 3 at a position close to the rotation center of the polishing surface table 2. The abrasive liquid L goes into the space between the polishing pad 3 and the semiconductor wafer S.
The adhesion film 8 is a polyurethane or polyester resin sheet of about 0.5 mm in thickness. The surface of the adhesion film 8 to contact the semiconductor wafer S is formed with minute sucker-like holes 8A, whereby the adhesion film 8 is given a function of holding the semiconductor wafer S by suction to prevent it from moving on the polishing head 5 during a polishing operation. Further, since the adhesion film 8 is elastic, it can absorb asperity, if any, due to unevenness in the thickness of the semiconductor wafer S or dust or the like sticking to the back surface of the semiconductor wafer S, thereby allowing the subject surface of the semiconductor surface to be polished uniformly.
To polish the semiconductor wafer S as an object, the semiconductor wafer S is held by the adhesion film 8 by water adhesion with a subject surface Sa placed outside (i.e., down). As shown in FIG. 1, the polishing head 5 is lowered in the direction indicated by arrow Z by operating the cylinder 7 in the state that the semiconductor wafer S is held in the above manner, whereby the semiconductor wafer S is pressed against the surface of the polishing pad 3 on the polishing surface table 2 with predetermined pressing force via the rotary shaft 6. The polishing surface table 2 is rotated in the direction indicated by arrow Ra at the above-mentioned rotation speed, and the polishing head 5 holding the semiconductor wafer S is rotated in the direction indicated by arrow Rb at the above-mentioned rotation speed. The abrasive liquid L is supplied to the surface of the polishing pad 3 through the abrasive supply hose 10. Chemical mechanical polishing (hereinafter abbreviated as xe2x80x9cpolishingxe2x80x9d) is thus performed on the subject surface Sa of the semiconductor wafer S by means of the polishing pad 3 and the abrasive liquid L.
Although during the polishing operation the rotation of the polishing head 5 and the polishing surface table 2 produces force of making the semiconductor wafer S go out of the surface of the polishing head 5, the retainer ring 9 prevents the semiconductor wafer S from doing so.
During a polishing operation, particularly immediately after its start, polishing friction resistance occurs between the semiconductor wafer S and the polishing pad 3, whereby shearing stress occurs between the semiconductor wafer S and the adhesion film 8. The holding force of the adhesion film 8 acting on the semiconductor wafer S is not necessarily uniform along the surface because of, for example, unevenness in the size and the density of the sucker-like holes 8A; there may exist portions where the holding force is strong and portions where it is weak. As the shearing stress increases, it is more concentrated on the portions where the holding force is strong, to cause very small distortions (asperity), which in turn cause unevenness of polishing in the subject surface Sa of the semiconductor wafer S.
If the polishing resistance is further increased and becomes so strong as to exceed the holding force of the adhesion film 8, the semiconductor wafer S is moved to collide with the retainer ring 9. Collisions may cause minute asperities on the surface of the semiconductor wafer S, to lower the uniformity of polishing. The semiconductor wafer S maybe broken by impact, or the retainer ring 9 may be destroyed by impact-induced fatigue failure, in which case the semiconductor wafer S goes out of the polishing head 5 and the polishing operation is disabled.
The present invention has been made to solve the above problems in the art, and an object of the invention is to increase the adhesion force by suction for holding an object of polishing such as a semiconductor wafer, to thereby provide a polishing apparatus which can polish an object with a high degree of uniformity in its subject surface.
To attain the above object, the invention provides a polishing apparatus for polishing a subject surface of a thin-plate-like object that is held by a polishing head with a polishing pad that is mounted on a surface of a polishing surface table, comprising sucking means provided in the polishing head; and a holding film provided on an object holding surface of the polishing head, the holding film being elastic and capable of passing air through itself.